Extensible non-combustible paper



Sept. 8, 1964 r 7 s. CLUETT 3,143,108

EXTENSIBLE NON-COMBUSTIBLE PAPER Filed Oct. 29, 1962 24% FIG. 3

BREAKING LENGTH METERS o l r I i I O 2 4 6 8 MAC E DIRECT'ON STRENGTH' INVENTOR. FIG. I

SANFOR D L. CLUETT BYKWZ AJ United States Patent 3,143,108 EXTENSIBLE N 0N -CUMBUSTHBLE PAPER Sanford L. Cluett, Troy, N.Y., assignor to Clupak, Inc, New York, N.Y., a corporation of Delaware Fiied Oct. 29, 1962, Ser. No. 233,583 6 Claims. (Cl. 162155) This invention relates to an extensible web formed of inert and substantially non-combustible material. More particularly, this invention provides an extensible web of asbestos having improved physical properties such as toughness, tear, smoothness, stretch, and flexibility giving the inert material greater utility without sacrificing its other useful properties.

Asbestos is Well-known as a relatively inert and noncombustible mineral which exists as bundles of different length fibers each of which has a subrnicroscopic diameter. The inert and non-combustible properties of asbestos make it useful in many cases where heat resistant, fire resistant, or fire proof construction material is needed and also where a non-corrosive insulation or insect-proof material must be used. The asbestos fiber may be formed into a usable sheet by selecting the long fibers, forming a thread and weaving the thread into a fabric web; or it may be formed into a paper sheet by selecting the short fibers and forming a water-laid asbestos web much in the same way cellulose paper web is formed but at much slower machine speeds. As the price of asbestos fiber is determined to a considerable extent by the length of the fiber, and as weaving is costly, an asbestos paper web formed of water-laid short fibers is of relatively low cost so that it can advantageously be used wherever its physical properties permit. In many instances, an asbestos paper web has been formed principally of asbestos dust (for example extremely short fibers 60% of which may pass a 100 mesh screen) held together by an adhesive or binder. The well-known disadvantages of such an asbestos paper web are that it is brittle and that it is subject to flexural cracking and failure when formed around short radius bends. Also, asbestos paper heretofore has been objectionably low in toughness properties, that is, in energy absorption capacity and in its tear and stretch characteristics. These poor physical properties increase the care required in applying the asbestos paper to desired uses and they increase the protection which must be provided to prevent rupture and the loss of its desirable properties which a rupture would cause. To provide some additional strength and fiexural resistance longer asbestos fibers are used in varying quantities with the asbestos dust, and certain impregnating grades of asbestos paper contain very high percentages of such expensive long fibers. Glass fibers and other inert or relatively inert and relatively heat resistant fiber are also added to mixtures of asbestos fibers of varying lengths for varying end uses. It will be apparent that increased percentages of long fiber asbestos or the addition of glass or other fibers to an already expensive blend of long fiber asbestos results in high cost which in many instances has been incurred only to give to the asbestos paper sufiicient strength and flexibility to withstand conversion into some end product wherein the long asbestos or reinforcing fibers make no continuing contribution. This is true for example in the case of asbestos boxes which after formation as paper webs are impregnated with resins or other compounds in liquid form and are then cured to set the resin or are laminated into multi-ply boards or blocks or other shapes which, in turn, are impregnated or further impregnated and are then treated to cure the impregnating or further impregnating materials or both. Electrical insulation panels or motor or generator parts as well as friction elements such as brake bands or blocks or clutch plates are frequently made by impregnating and laminating asbestos paper, the strength and fiexural resistance of which, as a paper, is important in the conversion thereof into the end product but may be of little importance in the end product.

In other instances asbestos paper is made from a mixture of short-fiber asbestos, including a minor or major percentage of fibers which would be classified as asbestos dust, with a binding material such as starch or a resin or other organic material or a mineral binder such as sodium silicate, and various quantities of organic fiber. The organic fiber may be used in some instances merely because it is cheaper than asbestos or in other instances to impart some desirable characteristic to the mixture or to the paper or both. For example cellulosic fiber in the form of rag fiber, chemical Wood pulp, mechanical wood pulp or reclaimed paper or cardboard stock is included in various asbestos paper for various purposes. Also synthetic fibers, usually staple fibers of some suitable length, made from polyamides (nylon), polyesters (Dacron), and similar materials have been used. Some of these organic fibers impart some additional tensile strength (dry or wet or both) flexural resistance, dimensional stability and the like to the resultant asbestos papers. Some of these organic fibers facilitate formation of the paper web from an aqueous suspension as by imparting greater freeness, that is more rapid drainage of water from an aqueous suspension, to an aqueous suspension of asbestos fiber. The short asbestos fibers and asbestos dust in the absence of some longer fibers will drain very slowly from an aqueous suspension making the formation of paper webs therefrom a very slow operation when carved out upon a web-forming apparatus such as a Fourdrinier papermaking machine or a so-called wet of cylinder paperrnaking machine. However, when longer fibers, having good drainage characteristics, such as cellulose fibers in the form of rag stock or chemical pulp, are added to the asbestos mixture the drainage rate or freeness is increased with attendant decrease in total cost of the asbestos paper.

Asbestos paper of commercial grades presently in use may contain anywhere from about asbestos fiber up to more than 99% asbestos fiber as constrasted with the organic materials such as adhesives, binders, cellulosic fibers, synthetic fibers or any desired mixture thereof, which the asbestos paper may also contain.

The presence in such asbestos papers of increasing quantities of organic or other materials which have a lower combustion point or which have lower heat resistance than the asbestos therein will result in generally proportionately lower heat resistance or fire resistance of the paper itself. This is fully recognized in the industry and a classification system has been set up on the basis of the organic content of the asbestos paper or end product made therefrom. Many useful papers and products need only to resist temperatures in the order of say 400 F. and such papers may be made from a mixture of about 75% to asbestos and about 25% to 20% organic materials. At the other end of the scale the papers or products may be called upon to resist temperatures of about 900 F. or more and in such cases a very low percentage of organic material is tolerated.

In some cases the paper or products made therefrom are subjected to pyrolysis to even further reduce the organic content thereof.

In all cases, however, asbestos papers which are commercially known as such and which contain about 75% or more asbestos, irrespective of the fiber size thereof, are relatively weak and brittle (when compared to a cellulosic paper) and it frequently has been the case that much more asbestos, and in particular more asbestos fiber of relatively great fiber length has been used in a particular product because of strength, toughness, stretch or flexural requirements than would be necessary for the heat resistance or fire resistance demanded of the product.

It has been taught by Cluett, US. Patent #2624145, that a web of cellulosic fibers can be modified to improve its physical properties by subjecting a partially dried, cellulosic paper web, while its fibers are in plastic condition, to compressive forces in a direction parallel to the surface of the paper web while the paper web is held under considerable pressure in a direction perpendicular to the Web surface. Because of the natural adhesive bonds between the cellulose fibers and their fibrils that are the product of finely beaten pulp, the cellulose fibers when distorted, flexed and crowded together in a space between the faces of the web impart a marked controllable and useful extensibility to the cellulose web. Additionally, it is thought in the paper art that the cellulose fibers also are held cementitiously together by molecular attractive forces which may be hydrogen bonds or Van der Waals forces made possible by the extreme intimate contact of the fibrillar cellulose material. This dual bonding of fibers and fibrils accounts to a great extent for the visco-elastic nature of cellulose paper.

The asbestos fiber on the other hand lacks the fibrils of the wood cellulose fiber and it lacks the natural adhesive bonds or hydrogen bonds of cellulose fibers. Therefore, asbestos fibers alone or in mixtures with cellulosic or other fiber in the commercial range discussed above for the manufacture of asbestos paper are unable to bind themselves per se into a continuous web having any usable strength. It is necessary in forming any such asbestos paper web to add a small amount of adhesive binder, for example about 1 to 5% by weight of starch to bind the asbestos fibers together. This percentage of starch to other binder, is kept as low as possible to retain the heat resistant or non-combustible properties and inert character of the asbestos in the final web product. Because asbestos is a mineral, it was heretofore thought that asbestos fibers would break if locally flexed as taught by Cluett. Also, because of the lack of natural adhesive and of hydrogen bonds and because of the large amount of dust which has been used to form the asbestos web paper, it was thought that the asbestos Web could not be modified to produce any significant change in its toughness and stretch characteristics. This was additionally thought to be true because the starch bond holding the asbestos fiber compacting or until heat was applied for drying the web. This completely different character of the asbestos web led those skilled in the art to believe that any compacting accomplished by modifying the asbestos web in accordance with the teachings of the Cluett patent would produce only a permanent shrinkage or compacting of the asbestos web and would not result in any recoverable extensibility 0r improved tensile strength, toughness or flexibility after drying. Early efforts along these lines appeared to verify this assumption. The presence in some asbestos papers of some relatively small amount, say from 19% or less to about 20% to of other fiber such as glass fiber, rock wool fiber, cellulosic fiber or synthetic fiber or mixtures thereof do not serve to materially alter the characteristics of the essentially asbestos fiber mixture inso far as the nature of the fiber bonding is concerned.

According to this invention, it has now been discovered that in spite of the different character of the asbestos fiber, the lack of any natural adhesives or internal bonding per se between the virgin asbestos fibers, and the fact that it is desirable to form an asbestos web with as little expensive long-fiber asbestos as possible with a substantial proportion of a non-combustible dust, it is possible to produce an inexpensive asbestos web paper with extensibilities far in excess of the primitive extensibility of the asbestos web. The presence or absence of other fiber, including cellulose fiber has little, if any effect upon the efficacy of the present invention in connection with any of the usual commercial forms of asbestos paper. At the same time, it is possible to improve the surface appearance and smoothness and to materially increase the toughness, tear and flexibility of the asbestos formed web or paper.

The modification of the asbestos web may be accomplished by the same type machinery on which the cellulose web taught by Cluett US. Patent No. 2,642,245 is modified. It has been discovered that the fibers of the asbestos web may be locally flexed without breaking when subjected to compressive forces parallel to the web surface at a time when the web is water-laid and still in a plastic state and that the high percentage of dust does not limit this extensibility. On the contrary, even though this dust would pass a mesh screen, the dust particles are in reality small bundles of the extremely short fibers of microscopic diameter. While this dust appears to be merely a filler, the fibers thereof do appear to interlock physically, when subjected to compressive forces and thereby aid in producing the improved properties of the asbestos web. The asbestos paper web when modified by the present process possesses a usable stretch without rupture which permits the new asbestos web to be folded or wrapped around sharp corners, flexed and distorted, and in addition, the web has a greater toughness than present asbestos paper.

It is therefore an object of the present invention to provide an extensible web of inert material and a method of making it.

Another object of the present invention is to provide an extensible web of asbestos fibers and asbestos powder and an inexpensive method of making it.

Another object of the present invention is to provide an asbestos paper web having improved properties of toughness, tear, smoothness, stretch, and flexibility.

Other objects and advantages of the present invention will be apparent from the following description and drawing illustrating an example of the practice of this invention in which drawing:

FIG. 1 is a stress-strain diagram showing the improved toughness characteristics of an asbestos web modified in accordance with the present invention;

FIG. 2 is a diagrammatic side elevation of simple apparatus useful in performing the compressive contracting steps or steps employed in producing the present product; and

FIG. 3 is a diagrammatic side elevation of apparatus which may be employed in conjunction with the apparatus of FIG. 2.

The new and useful product and method of the present invention will be first explained by referring to an example of the production thereof by use of apparatus such as shown in FIGS. 2 and 3.

The apparatus of FIGS. 2 and 3 comprises rolls 10, 11, 12, a heated driven roll 13 and a thick belt 15 with a contractible or elastomeric surface layer, preferably of rubber with a durometer hardness sufficient to prevent creping of the paper web. This belt may be formed of elastomeric material such as natural rubber of rubber substitutes, and preferably in a continuously running belt. It preferably has a strong relatively inextensible layer faced with a readily extensible and contractible surface layer of any suitable material which has a smooth elastic or stretchable and contractible surface of sufiicient hardness and extensibility.

The rolls and 13 are adjustably movable toward and from each other so as to properly nip the belt between them where the belt passes from the roll 10 and to the roll 13. The roll 12 is spaced away from the roll 13 sufficiently to give the belt a short straight run from the roll 13 to the roll 12. As the belt passes from the roll 10 to the roll 13, the outer surface or layer of the rubber belt which is convexly curved and stretched while passing around the roll 10 becomes concavely curved and compressed when passing over roll 13 and accordingly such previously outer layer shortens in length.

A moist paper web fed over roll 10 in firm contact with the convex surface of the belt will adhere frictionally, to this belt surface as it passes between the belt and roll 13 where this shortening of the belt surface is taking place by reversing its curvature. The paper web is forced by pressure into such frictional contact with the contracting surface of the belt that the contracting belt surface tends to shorten the adhering paper web and compress it longitudinally in a direction parallel with the surfaces of the web. The nip on the belt indents the belt and causes the convex belt surface at the nip to momentarily move faster than the body of the belt is travelling, which serves to stretch the belt surface and web just as they pass into the heated roll where contraction occurs of the belt surface and adherent web. This increases the amount of compression of the web. The roll 13 must be accurately machined, ground and finished to a true cylinder of smooth periphery. It is heated not only to cause a partial .drying of the web but also to lower the coefiicient of friction between the drum and the moist Web, while at the same time heating the water contained in the web and thus causing a softening and increased flexibility of the fibers as they are crowded together by the contraction of the belt surface. The resulting loss of water in the web in effect makes room between the fibers during further shortening of the web for further crowding together of the fibers of the web as may be desired.

The coefiicient of friction between the wet web and the heated drum 13 is most effectively reduced at drum temperatures at least above 212 F. The coefficient of friction of the surface of the roll 13 is relatively low as compared to the coefficient of friction of the web-contracting surface of the belt 15. Thus, under the influence of the contracting belt surface, the paper web tends to partake of such contraction of the belt surface and slide with relation to the roll surface. During the longitudinal compression of the paper or web, the tension in the rubber belt is maintained sufficiently high. Thus, with the selected hardness of the belt surface, the pressure between the rubber belt 15 and the roll 13 prevents the paper from creping and folding upon itself and keeps the surfaces of the paper plain, smooth and parallel. As the paper web is compressively shortened the individual fibers of the paper including dust fibers which lie generally lengthwise of the web or in the direction of shortening are compressively distorted laterally of themselves and slide along one another within the body of the web.

Elastic rubber, natural or synthetic, is preferred as the material for the contracting surface, such as that of the belt 15, because of its elasticity and ability to withstand strong tension and heavy pressure transverse to its surface. This allows a relatively large and effective contraction of the surface to take place without allowing the paper to crepe in response to such contraction. Elastic rubber is also preferred for its continuously smooth surface and for its ability to grip the paper frictionally to the extent of compressing the paper longitudinally in the presence of the heavy pressure exerted between the belt and the roll 13 for prevention of creping. In the apparatus as shown, it is preferable that the rubber layer be reinforced along one face by comparatively inextensible material such as heavy canvas or layers of strong cords. The

necessary high tension in the belt then can be maintained and the rubber or elastic layer or surface of the belt then can expand and contract uniformly while passing over the roller 10 and the heated driving roll 13 with the relatively inelastic belt layer in contact with the roll 10. In some cases it may be advisable to pass the web through the apparatus one or more additional times, or to provide two or more sets of apparatus in series. Additional passes provided in some cases will provide greater stretchability to meet specific demands.

In the instant invention, the apparatus of FIG. 3 was employed in conjunction with the apparatus of FIG. 2. The FIG. 3 apparatus continuously received the compressed asbestos web from the apparatus of FIG. 2 and dried the web in its compressed condition without applying material tension to it. As shown in FIG. 3, the drying apparatus includes a moisture-permeable felt belt 21 which passes over an intake roll 22 and thence onto and around a heated drum 20, holding the paper web against the drum during drying. To be strongest, the asbestos web should be a freshly laid web as the starch bond is largely irreversible. However, if it is no possible to use a freshly laid web, a dried web may be re-wet and then modified by the above described apparatus in order to obtain an extensible web with many of the advantages of this invention.

It is usual in forming an asbestos paper web to use rotating vacuum drums onto which the asbestos fibers and asbestos dust are water-laid. The desired thickness is obtained by passing the web over one or more rotating vacuum drums to build up the newly formed web in a laminating fashion to the desired thickness. The web so formed passes to the modifying and drying machinery such as described above Where it receives its extensible properties.

The improved physical properties of the present asbestos paper web can be shown by a comparison with a conventional water-laid asbestos paper formed in the usual manner. The modified asbestos paper, in accordance with this invention, was re-wet after drying and then modified according to the invention to improve its physical properties. The modified web sample contained 3% starch as an adhesive binder and the average asbestos fiber length was as follows (closest Whole percentage used) Table I Fractions: Fiber length (inches) 2% .09. 2% .10. 17% .103. 18% .04.

60% Asbestos dust (passed mesh screen). FIG. 1 shows a comparison of the stress-strain characteristics of a control sample as compared with the presend modified asbestos paper sample. The control sample curve A had the same composition as the modified sample, curve B, except for its compression to provide its extensibility. As FIG. 1 shows, the modified sample has substantial extensibility in the machine direction and without rupture well in excess of its primitive strain limit.

The graph shown in FIG. 1 also illustrates other basic mechanical differences between the two asbestos papers. Curve A is for conventionalasbestos paper having a basis weight of 11.2 pounds per 100 square feet. Curve B is for extensible asbestos paper which after compression has a basis weight of 12.5 pounds per 100 square feet. The difference in the initial slope of the two curves is a direct measure of the flexibility or to use a more common term, drape. The. curve having the greater slope also has the greater stiffness. It will be seen that curve B has the smallest slope and therefore, has the greater flexibility as compared to the sample asbestos paper shown in curve A. An integration of the area under the curve is a representation of the work required to rupture the paper web. A comparison of the work required in inch pounds per square inch of surface area is shown in the following table as the toughnesss of the two samples. It will be seen in Table II that the work required to rupture the modified asbestos paper in the machine direction is about ten times as great as the work required to rupture the control sample paper:

Table II Control Modified Sample Sample Caliper, inches 0. 0100 0. 0097 Breaking Length:

1 NOTE.MD refers to the machine direction, or direction in which the paper was run through the paper machine when it was formed and through the compression process during compression. CD is the cross direction.

A comparison of other important physical properties of the control sample and of the modified sample also are shown in Table II. The basis weight, which is the weight of 500 sheets, 25 inches by 38 inches, increased indicating that the density of the modified sample was increased while at the same time the thickness as indicated by the caliper reading was actually slightly decreased. The modified sample was modified only in the machine direction, that is, the direction in which the paper travels through the paper forming machinery. This modification produced a 17.8% stretch of the paper web in the machine direction as compared to a 0.5% stretch in the control sample in the machine direction. A slight increase in stretch was also obtained in the cross direction in the modified web.

When the water-laid web is modified as described above, the extensibility or stretch may be selectively provided to obtain substantial extensibility even up to 100% without rupture. The stretch that may be provided in a modified Web is very useful when the asbestos paper is to be formed around an irregular shape. Generally, the higher the stretchability that is provided to the web, the higher the toughness of the modified web. This improved property increases the usefulness of the web materially and allows a higher selling price.

Table 11 also indicates a reduction in the machine direction in the stiffness of the modified sample. The stiffness measurements are recorded in Gurley Stitfness Units in which the higher numbers have the greater stiffness. Lack of stiffness is important in the modified asbestos paper since it is one measure of the ability of the paper to take a radius bend or to form around an object easily Without failure or rupture.

The breaking length, that is, the length of itself which the paper will support without breaking, decreased following modification of the paper web, as seen in Table II. This reduction was to be expected since generally higher tensile strengths accompany higher stiffness.

The smoothness of the modified sample of asbestos paper is shown to have increased on both the drum and the blanket sides of the paper. The smoothness measurements are the Chapman measurements in which the higher reading indicates a higher smoothness. This smoothness is important in such end use applications as printing and as the modified asbestos web is devoid of folds and pleats, it may be printed upon easily.

The tear measurements were made by standard procedures of the Technical Association of the Pulp and Paper Industry and indicate an increase in the force to produce a tear in the machine direction. It was not pos sible to obtain cross direction (CD) tear values because the paper would not tear anything resembling a straight line in that direction. Upon starting a tear in the cross direction, the tear line immediately turns and follows a line in the machine direction.

The invention has been described as employed in the production of a stretchable asbestos paper web, because asbestos is found in nature in adequate quantities for commercial utilization, is relatively low in cost, and is an ideal fire resistant or heat resistant material which may be used in the production of papers with or without other fibers such as cellulose fibers, glass fibers, synthetic staple fibers and the like to make paper and products having predictable heat resistant and fire resistant properties. Asbestos fibers are freely flexible when wet and are not brittle after compaction so the web can be Wrapped around objects to be protected from heat loss or exposure. Also, asbestos powder or dust is available at low cost as a filler. However, some of the advantages of the invention can be obtained by using other mineral or inert fibers if they are quite flexible and are not brittle. The dust may be any mineral, metal or inert non-combustible dust, but since asbestos in both the dust and short fiber form is so inexpensive and satisfactory, its use at present is preferred. The proportion of dust in the mixture can be varied considerably depending on the surface desired for the face of the web.

Starch is a readily available and a relatively inexpensive adhesive that is effective in low amounts as the binder. Starch also becomes sticky or plastic when wet. While it can be used in large amounts, it is effective in amounts as low as from about 1% to 5% by weight in this mix ture. Other adhesives can be used if they are plastic and have adhesivencss when heated or wet. For example sodium silicate is widely used in the manufacturing of asbestos papers of various grades. Such other adhesive binders may be thermo-sensitive adhesives such as phenol formaldehyde compounds for example.

It will be understood that various changes in the details, materials, steps and proportions of components in the web, may be made by those skilled in the art Within the principle and scope of the invention, as expressed in the appended claims.

Also, it will be understood that the use in the claims of the expression an asbestos web consisting essentially of a mixture of asbestos fibers, asbestos dust, and an amount, not exceeding 5% by weight of adhesive hinder or the expression consisting of asbestos fibers and asbestos dust in the range of to 99 percent by weight of the web is not intended to exclude from the scope of such claims asbestos paper webs of the types generally accepted in the industry as asbestos paper which may contain some cellulosic or other organic fibers or other fibers such as glass or rock wool fibers in quantities not ordinarily exceeding about 20 to 25 percent by weight of the dry paper. As pointed out above the presence of such other fibers does not substantially alter the characteristics of the absestos paper insofar as the present invention is concerned.

The present application is a continuation-in-part of my application Serial No. 766,563, filed October 10, 1958, for Extensible Non-Combustible Paper, now abandoned.

I claim:

1. The process of producing a relatively dense asbestos paper web of high toughness having a minimum amount of adhesive binder, which comprises confining an asbestos web consisting essentially of a mixture of asbestos fibers, asbestos dust, and an amount not exceeding 5% by weight of an adhesive binder, while said binder is plastic, frictionally against a contractible surface with a perpendicularly applied pressure, and while the web is so confined contracting said surface with resultant compression of the web in a direction parallel to its faces and crowding together of the asbestos fibers, and then causing said adhesive binder to harden in the compressed condition of the web, whereby a cohesive, relatively tough asbestos Web is formed having a minimum amount by Weight of adhesive binder.

2. The process of producing a relatively dense asbestos paper Web of high toughness having a minimum amount of adhesive binder, which comprises confining an asbestos web consisting essentially of a mixture of asbestos fibers, asbestos dust, and an adhesive binder, while said binder is plastic, frictionally against a contractible surface with a perpendicularly applied pressure, and while the web is so confined contracting said surface with resultant compression of the web in a direction parallel to its faces and crowding together of the asbestos fibers, and then causing said adhesive binder to harden in the compressed condition of the web, whereby a cohesive, relatively tough asbestos web is formed having a minimum amount by weight of adhesive binder.

3. The process of producing a relatively dense asbestos paper web of high toughness having a minimum amount of adhesive binder, which comprises confining an asbestos paper web consisting of a mixture of asbestos fibers and asbestos dust in a range of 85 to 99 percent by weight of the web, together with a minimum percentage by weight of adhesive binder, while said binder is plastic frictionally against a contractible surface with a perpendicularly applied pressure, and while the web is so confined contracting said surface with resultant compression of the web in a direction parallel to its faces an crowding together of the asbestos fibers, and then causing said adhesive binder to harden in the compressed condition of the web, whereby a cohesive, relatively tough asbestos web is formed having a minimum amount by weight of adhesive binder.

4. An asbestos paper produced in accordance with the process set forth in claim 1.

5. An asbestos paper produced in accordance with the process set forth in claim 2.

6. An asbestos paper produced in accordance with the process set forth in claim 3.

References Cited in the file of this patent UNITED STATES PATENTS 1,971,162 Novak Aug. 21, 1934 2,015,416 Toohey et al Sept. 24, 1935 2,133,693 Greider et al Oct. 18, 1938 2,517,724 Schuh Aug. 8, 1950 2,567,558 Grieder et a1 Sept. 11, 1951 2,593,125 Easton et al Apr. 15, 1952 2,624,245 Cluett Jan. 6, 1953 2,652,325 Novak Sept. 15, 1953 2,708,982 McGutf May 24, 1955 2,773,764 Park Dec. 11, 1956 OTHER REFERENCES Clark: TAPPI, Dec. 24, 1942, pages 328-333. 

1. THE PROCESS OF PRODUCING A RELATIVELY DENSE ASBESTOS PAPER WEB OF HIGH TOUGHNESS HAVING A MINIMUM AMOUNT OF ADHESIVE BINDER, WHCIH COMPRISES CONFINING AN ASBESTOS WEB CONSISTING ESSENTIALLY OF A MIXTURE OF ASBESTOS FIBERS, ASBESTOS DUST, AND AN AMOUNT NOT EXCEEDING 5% BY WEIGHT OF AN ADHESIVE BINDER, WHILE SAID BINDER IS PLASTIC, FRICTIONALLY AGAINST A CONTRACTIBLE SURFACE WITH A PERPENDICULARLY APPLIED PRESSURE, AND WHILE THE WEB IS SO CONFINED CONTRACTING SAID SURFACE WITH RESULTANT COMPRESSION OF THE WEB IN A DIRECTION PARALLEL TO ITS FACES AND CROWDING TOGETHER OF THE ASBESTOS FIBERS, AND THEN CAUSING SAID ADHESIVE BINDER TO HARDEN IN THE COMPRESSED CONDITION OF THE WEB, WHEREBY A COHESIVE, RELATIVELY TOUGH ASBESTOS WEB IS FORMED HAVING A MINIMUM AMOUNT BY WEIGHT OF ADHESIVE BINDER. 